In the extrusion of thermoplastic materials through a crosshead, it is common practice to pump the molten thermoplastic material into a supply passage and to force the molten thermoplastic material out through a die opening. The die opening may be of any desired cross sectional shape.
When a tubular layer of thermoplastic material is to be molded, it is necessary to provide a core or a mandrel inside the die opening. When this tubular layer of thermoplastic material is electrical insulation on a wire, then the wire, or the wire plus a previously extruded layer of thermoplastic material, is the core.
When a thermoplastic tubing is to be extruded, then a steel mandrel determines the inside diameter of the layer of thermoplastic material; or, in the case of coextruded tubing having a plurality of layers, the outside surface of an inner and previously extruded tubular layer of thermoplastic material is the core.
Further, it is common practice to extrude a thermoplastic material into a tubular shape, and then to process the tubing into bottles, or other containers, by pinching off a portion of the tubing, by placing the portion of tubing into a mold, and then by blowing the tubing into the shape of the mold.
Since the molten thermoplastic material must be supplied through a supply passage that opens on one side of the core or mandrel, the flow of thermoplastic material must flow transversely inward and then around the core or mandrel to form an annular flow of material that can be extruded longitudinally.
It is common practice to divide the flow of thermoplastic material into two streams that each flow half way around the longitudinal axis of the crosshead, and that join distal from the supply passage.
Some types of thermoplastic materials tend to develop a weld line where the separate branches of thermoplastic material join. This weld line exists in the annular and longitudinal flow of the thermoplastic material, in the finished tubing, and in a blown container made from the tubing.
Weld lines are detrimental for two reasons. One is that they are unsightly in what otherwise is a clear and glass-like container. The other reason is that they weaken the tubing; and so the tubing may burst at the weld line while being blown into a container.
Therefore, it is important to be able to produce extruded tubing without weld lines without regard to the type of thermoplastic material being extruded.
The problem of weld lines in tubing extruded from thermoplastic materials is even more critical for tubing that is to be used for fabricating containers for certain uses; because, for these particular uses, a homogeneous tubing of a single thermoplastic material is not the most desirable.
For instance, in the food processing industry, where the preservation of taste and the prevention of deterioration through contact with air is essential, and where the cost of the containers is quite competitive, it is economically imperative that bottles, jars, and tub-shaped containers be made with layers of various thermoplastic materials that differ in physical characteristics and cost.
Thus a container for the food processing industry may have a layer of thermoplastic material for the inside of the container that will not impart a foreign taste to the food product and that will conserve freshness by excluding air, a similar layer of thermoplastic material for the outside of the container, and an inner layer of a more economical thermoplastic material.
These multilayered plastic containers are blown from tubing that consists of several and sometimes as many as six, or more, layers of thermoplastic material.
Often, the containers are made of thermoplastics that are transparent; and uniform transparency is important to the food processor; and a weld line in any of the layers of thermoplastic in the extruded tubing results in a blemish that is deterimental to the appearance of the finished containers.
Some types of thermoplastic materials, particularly polypropylene which is ideal for the inner and outer surfaces of containers that are used by the food processing industry, are difficult to extrude without a weld line. Thus, it is extremely important to have a crosshead that will extrude multilayered tubing of various types of thermoplastic materials without forming a weld line in any of the layers.
In U.S. Pat. No. 3,355,769, Folgelberg disclosed a crosshead in which the flow of plastic material entered tangentially around the mandrel to prevent the weld line.
However, since some thermoplastic materials, such as polyvinyl chloride, degrade as a function of time and temperature, it is important that the molten thermoplastic material proceed through the crosshead in a minimum time, with a uniform velocity, and without stagnant areas.
In contast to these flow requirements, which dictate distribution passage size and shape, the distribution passage of Folgelberg is relatively large. Also, his distribution passage is uniform in cross section; so that the velocity of the molten plastic would be much too slow at a point diametrically opposite of the supply passage.
Wheeler, Jr., et al., in U.S. Pat. No. 4,280,801, show and describe the supplying of molten thermoplastic material to the die by means of a supply passage, a first distribution branch that is connected to the supply passage and that partially encircles the longitudinal axis in a first direction, and a second distribution branch that is connected to the supply passage and that partially encircles the longitudinal axis in the other direction and that joins, or abuts, the first branch diametrically distal from the supply passage.
The first and second distribution branches of Wheeler, Jr., et al., are proportioned in cross sectional area, being a maximum at the supply passage and reducing to a minimum where the branches join distal from the supply passage.
By this progressive reduction in cross sectional area in the flow paths, stagnation in the flow of the molten plastic has been substantially eliminated, a reduction in the time that any of the molten plastic is in the crosshead has been achieved, and a reduction in heat-caused degradation of the thermoplastic material has been achieved.
However, even though Wheeler, Jr., et al. achieved their objective of eliminating stagnant areas in the flow path, a weakness of crossheads that utilize their type of feed passages is that there is the tendency to develop weld lines in the extruded thermoplastic tubing. These weld lines occur at a circumferential location on the extruded tubing that corresponds to the point where the separate distribution branches join distal from the supply passage.
It is believed that the weld line is caused by a film of degraded material on the surface of the two branches of molten thermoplastic that meet distal from the supply passage; and so mixing of these two flows, to disperse the degraded film, should eliminate the weld line.
The prior art teaches the use of static mixing in the extrusion of thermoplastics. In U.S. Pat. No. 2,669,750, Keeney taught the use of static mixing to assure uniform color in extruded materials in which a dye is mixed with the powder of a thermoplastic.
Further, the prior art teaches the use of static mixing to eliminate weld lines in extruded tubing. Larsen, in U.S. Pat. No. 4,173,446, teaches the use of static mixing to eliminate weld lines where several radial branches of thermoplastic are fed to the extrusion die for large diameter tubing.
However, the prior art has not taught apparatus or methods that eliminate weld lines to the extent that is needed for the production of multilayered tubing for use in producing blown containers for the food processing industry.
In contrast, the present invention provides apparatus and method for completely eliminating weld lines in multilayer extruded tubing made from thermoplastic materials.